A. In treatments using kilovoltage x-ray machines, doses were usually limited by the tolerance of ______.

B. Higher-energy x-ray beams are more ______.

A. skin

B. skin-sparing

A. X-ray beams 1 MeV or greater can be classified as ______ beams.

B. List 6 examples of machines that produce these high-energy beams 

A. megavoltage beams

B. 

(1) Linear accelerator

(2) Cobolt-60 unit

(3) Van de Graaff generator

(4) Betatron

(5) Cyclotron

(6) Microtron

[image]

What type of machine is this?

[image]

What type of machine is this?

The cyclotron consists of a short metallic cylinder divided into two sections, called the Ds.  The Ds are placed between the poles of a direct current magnet (producing constant magnetic field).  Charged particles are injected into a chamber (ion source) at the center of the Ds.  The magnetic field causes the particles to travel in a circular orbit.  

An alternating potential is applied between the Ds, the frequency of which is tuned such that as the particle is accelerated each time it passes between the Ds.

With each pass between the Ds, the particle orbits at a greater radius.  The correct energy can be obtained by extracting the charged particle at the orbital radius corresponding to that energy.

[image]

What type of machine is this?

In a microtron, electrons travel in circular orbits under the direction of a constant magnetic field.  At the end of each revolution, the electron is accelerated by an oscillating electric field as it passes through a microwave cavity, powered by a magnetron or klystron.

With each revolution, the radius of the electron's orbit increases as its energy increases.

Electrons are extracted with a movable deflection tube (made of steel to shield from the effects of the magnetic field).  Energy is selected by placing the deflection tube at the corresponding orbital radius.

The extracted electron beam can be guided to a treatment head similar to that of a linear accelerator.

[image]

What type of machine is this?

(1) Traveling waves require a terminating (dummy) load to absorb residual energy at the end of the tube to prevent reflection of the wave.

(2) Accelerators using stationary (standing) waves provide maximum reflection at both ends of the structure so that the combination of forward- and backward-traveling waves result in a standing wave.  A circulator is required to prevent backward reflections from reaching the power source.  Standing wave design is more expensive.

The Linac Power Supply

A. What type of power does the power supply deliver to the linear accelerator?

B. Which component of the machine first receives this power?

A. Direct current (DC) power

B. The modulator

The Modulator

A. Name two components of the linac modulator.

B. The modulator simultaneously delivers high-voltage pulses of DC to what two components of the linac?

A.

(1) The pulse-forming network

(2) A switch tube (hydrogen thyratron)

B.

(1) electron gun

(2) magnetron / klystron

The Magnetron

A. What is the function of the magnetron in a linear accelerator?

B. Briefly describe the operation of the magnetron.

A. The function of the magnetron is to produce high-frequency microwaves (~3GHz) to be used to accelerate electrons in the accelerator tube.

B. The magnetron is a cylinder containing a central cathode and an outer anode.  Resonant cavities are machined out of the anode.  A static magnetic field is applied perpendicular to the plane of the cavities.  A pulsed DC electric field is applied between the cathode and the anode.  Electrons are emitted from the cathode filament via thermionic emission.  These electrons interact with the perpendicular electric and magnetic fields, spiralling toward the resonant cavities and radiating energy in the form of microwaves.

The Klystron

A. What is the function of the klystron?

B. What are the advantages and disadvantages of the klystron compared with the magnetron?

A. Unlike the magnetron, the klystron does not generate microwaves.  It is rather a microwave amplifier, driven by a low-power microwave oscillator.

B. Klystrons are more expensive than magnetrons, but typically have a longer lifespan and are capable of delivering higher power levels (preferred as beam energy approaches 20 MeV or higher).

The Beam Transport System

A. What is the function of the beam transport system in a linear accelerator?

B. Name two components of the beam transport system.

C. When may it be necessary to redirect the electron beam before it strikes the target?  In this case, by what angle is the primary electron beam bent? 

A. The function of the linac beam transport system is to guide the electron beam from the accelerator tube to the treatment head.

B.

(1) bending magnets
(2) focusing coils

C. Production of higher-energy (>6 MeV) electrons requires an accelerator tube of greater length.  It is often not feasible to have the beam strike the target head-on, as the resulting x-ray distribution is highly forward-peaked; the length of the treatment head would add to the already long length of the machine.

To compensate, the electron beam can be bent at a suitable angle, most commonly 90º and 270º.

The Treatment Head

[image]

Label structures A-J in the diagram above.

A. x-ray target

B. primary collimator

C. flattening filter

D. scattering foil

E. carousel

F. ion chamber

G. secondary collimator

H. slot for beam modifiers

I. accessory mount

J. electron applicator

The x-ray beam emerging from the target of a linear accelerator is not uniform.

A. Why?

B. What is used to improve the beam uniformity?

C. Name two suitable materials for this device.

A. The high-energy electrons bombarding the linear accelerator target produce a forward-peaked x-ray beam.  The intensity towards the center of the beam is thus much greater than that towards the field edges.

B. A flattening filter

C. 

(1) Lead (most common)

(2) Tungsten

A. In electron therapy mode, the x-ray target is replaced by the _______.  What is the function of this device?

B. Describe the choice of material and thickness of this device.

A. The scattering foil is used to spread the pencil beam of electrons into a uniform fluence of electrons across the treatment field.

B. The scattering foil is a thin metallic foil, typically of lead.  A thickness is chosen such that the electrons will predominantly be scattered and not suffer bremsstrahlung interactions.

(1) The x-ray beam is shaped into a square field by the fixed primary collimator immediately after exiting the target and before it passes through the flattening filter.

(2) After the flattened beam passes through the ion chamber, it is again shaped by a movable secondary collimator.  The secondary collimator consists of two pairs of lead jaws capable of providing rectangular openings that correspond to field sizes ranging from 0 x 0 cm to ~40 x 40 cm, as projected at a standard SAD (typically 100 cm).  The collimator blocks move such that the block edge is always along a radial line to the target.

A. Name three beam parameters measured by the dose monitoring system.

B. When are these parameters measured?

C. What type of ion chamber is most commonly used?

A. The ion chamber monitors dose rate, integrated dose, and field symmetry.

B. The x-ray beam encounters the ion chamber after it has been flattened but before it passes through the secondary collimator or beam modifiers.  Thus, inaccuracies due to the accelerator, primary collimator, or flattening filter can be differentiated from those arising further downstream.

C. The ion chambers are typically transmission-type.  These are flat, parallel plate chambers that cover the entire beam.  Cylindrical thimble chambers have also been used. 

Describe the function and placement of the light localizing system.

As opposed to photon fields, which are collimated by movable jaws, electron beams are collimated by attachable cones of various sizes.  Because electrons scatter considerably from collimator surfaces, the effect of field size on dose rate is much greater than with x-rays.  Ensuring such a high accuracy of jaw position would be a burden.  

Because electrons scatter readily in air, the electron beam must be collimated close to the patient surface (usually 5 cm).

A. How is isocenter defined?

B. Describe isocentric treatment.

A. The isocenter is defined as the point of intersection between the gantry's axis of rotation and the collimator axis (which should correspond to the central axis of the beam).  This imaginary point is a fixed distance from the source, the source-to-axial distance (SAD).

B. The linear accelerator is designed and calibrated in such a way that the central axis of the beam is always focused on the isocenter.  In an isocentric treatment, the isocenter is 'placed' inside the patient at the desired location (rather, the patient is set up so that the point of interest within the patient coincides with the isocenter of the machine).  As a result, treatment beams emanating from any direction (gantry angle) will intersect at the desired point.

(1) High specific activity (Ci/g)

(2) High radiation output per unit activity

(3) Sufficient average γ-energy (1.25 MeV)

(4) Relatively low self-absorption 

A. How is the ⁶⁰₂₇Co source produced?

B. Describe the geometry of a typical Co-60 source.

C. How are the emitted β-particles filtered from the primary beam?

D. What percentage of beam intensity is the result of scatter components?  How does this scatter generally arise?

A. Co-60 is produced by the bombardment of stable Co-59 atoms with neutrons in a nuclear reactor.

B. The cobalt source is usually a solid cylinder placed with the circular end facing the patient.  The diameter of the circular end of the source is usually between 1.0 - 2.0cm.

C. Unwanted β-particles are absorbed by the source itself, as well as the stainless steel encapsulation.

D. ~10% of the total intensity of the beam comes from scatter.  The scatter consists mostly of lower-energy γ-rays produced by interactions of the primary γ-beam with the source, its capsule, the source housing, and the collimation system.

⁶⁰₂₇Co has a half-life of 5.26y.  

In >99% of disintegrations, a β^- of E_max=0.32 MeV is emitted and the daughter nucleus emits two distinct γ-rays (γ₁=1.17 MeV, γ₂=1.33 MeV) before reaching ground state (⁶⁰₂₈Ni).

In <1% of disintegrations, a β^- of E_max=1.48 MeV is emitted.  Here, the daughter nucleus only emits γ₂ before reaching ground state.

Average γ-energy ~ 1.25 MeV

A. What is the source housing for a Co-60 unit called?

B. What does this housing consist of?

C. Describe four mechanisms by which the source is switched on/off.

A. The sourcehead

B. The sourcehead is a steel shell filled with lead.  It consists of a mechanism for switching the source between on and off positions.

C. 

(1) Rotating wheel

(2) Horizontally-sliding heavy metal drawer

(3) Flowing mercury

(4) A shutter consisting of heavy metal jaws

A. Describe the collimation of a Co-60 unit.

B. What types of penumbra occur as a result of the Co-60 unit design?

C. How can these effects be minimized?

A. The most simple (and common) form of collimation in Co-60 units consists of two pairs of heavy metal blocks, each of which can be moved independently to obtain a rectangularly shaped field.  The distance from the source to the blocks is called the source-to-diaphragm-distance (SDD).

B.

(1) Transmission penumbra (if the block edges are parallel  to the central axis of the beam)

(2) Geometric penumbra (because the source is not a point-source)

C.

(1) The effects of transmission penumbra can be minimized if the blocks are designed so that the inner edges are always parallel to the edge of the beam.

(2) The effects of geometric penumbra can be minimized  by increasing SDD.  Additionally, heavy metal bars, called penumbra trimmers, can be placed in the penumbra region at distance > SDD to sharpen the field edges.  New SDD = source-to-trimmer distance.

A. What is the range of energy for proton beams used in radiation therapy?

B. What is the major advantage of high-energy protons and other heavy ions?

C. Describe the relationship between the range, atomic mass, and atomic number of a proton or heavy ion.

A. 150 - 250 MeV

B. The major advantage of using protons and heavy ions in therapy lies in their characteristic depth-dose curve: the dose deposited is approximately constant with depth until near the end of the particle's range.  Here, the absorbed dose peaks to a very high value and rapidly falls to zero.  This region of high dose is called the Bragg peak.

C. The range of a proton or heavy ion is proportional to A/Z².  Thus, heavier particles have shorter ranges than lighter particles of the same energy/nucleon (MeV/μ).

A. List the three types of pions (Π mesons) and describe the decay of each.

B. Why is the Bragg peak more pronounced for negative pions than for other heavy particles?

A.

(1) Π⁺ --> μ⁺ + v

(2) Π^- --> μ^- + v

(3) Π⁰ --> hv₁ + hv₂

B. The Bragg peak for Π^- is more pronounced because the additional effect of nuclear disintegration by Π^- capture, known as star formation.  Π^- capture results in the release of several other particles from the nucleus, including protons, neutrons and α-particles.